1. Field of the Invention
The present invention relates to apparatus for use in defiberizing papermaking stock. More particularly, the invention relates to extraction bedplates with specially shaped and contoured holes cut by laser energy or a fluid jet for use in pulping apparatus.
2. Background Art
Apparatus for pulping paper making stock is shown in Chupka U.S. Pat. No. 4,725,007, the disclosure of which is incorporated by reference. The apparatus shown in U.S. Pat. No. 4,725,007 includes a tub and a rotor mounted within the tub for inducing shear forces which serve to defiberize the stock. An extraction bedplate is positioned at the bottom of this tub, surrounded by a frusto-conical wall which serves as a funnel to direct the stock toward the bedplate. The preferred bedplate is disc-shaped, defining an upstream surface facing into the tub; a downstream surface facing oppositely from the upstream surface; and holes or apertures extending through the bedplate from the upstream surface to the downstream surface. The rotor is mounted near the center of the perforated bedplate and coupled to a motor for rotation about an axis normal to the upstream surface of the bedplate.
The holes extending through the extraction bedplate allow accepted fiber, that is, pulp which has been defiberized to a degree which is acceptable for further processing to flow out from the apparatus, while retaining larger, undefiberized particles and other solids in the tub. Conventional bedplates typically range from 24 inches (61 cm) to 96 inches (2.4 m) in diameter and are typically approximately ⅝ inch (1.6 cm) thick. Typically there are 4,000 to 5,000 holes in a 96 inch diameter plate with ⅝ inch holes. Since such holes are formed by conventional drilling processes, they have in the past been formed parallel to the axis of the bedplate with circular cross sections. The holes generally range from ⅛ inch (3.2 mm) to 1 inch (25 mm) in diameter.
Known extraction bedplates tend to be high maintenance items because of wear. Bedplates are exposed to harsh treatment from sand, metal objects and other debris contained within the stock. The typical clearance between the rotor and the bedplate is approximately 0.060 inch (1.5 mm) to 0.120 inch (3.0 mm). The stock is constantly pushed against, and drug along, the upper surface of the bedplate by the mechanical and hydraulic action of the associated rotor. The accepted fiber along with small contaminates which flow through the bedplate contribute to wear within the holes, particularly near the upper perimeters of the downstream edge portions of the holes.
Bedplates typically are manufactured from steel alloys resistant to wear and corrosion. Various stainless steels and 410 hard chrome steel have been used in forming bedplates. The 410 hard chrome steel is preferred because it is more wear resistant than the stainless steels. On the other hand, the 410 hard chrome steel requires heat treatment to harden the material to restore acceptable wear resistance after known machining and hole-drilling steps are performed. Once the heat treatment is performed, further machining is possible only with special tools in a slow and costly procedure. The heat treatment itself tends to warp the steel, so that additional manufacturing steps are required to straighten the bedplate.
The defibering characteristics of a given bedplate are dependent to a large degree on the surface indentations defined by the upper edges of the individual holes. More particularly, the paper making stock flows over the upstream surface of the bedplate during operation of the pulping apparatus. Hydraulic shear is generated near downstream side edges (that is, edges facing the oncoming stock now) formed at the intersections of the holes with the upstream surface of the bedplate. This hydraulic shear acts to break up relatively large, undefiberized particles. Increasing the number of such downstream side edges increases the amount of the hydraulic shear, thus improving the efficiency of the pulping apparatus.
Therefore, there remains a need in the art for extraction bedplates providing improved efficiency and wear resistance. Additionally, there remains a need for improved methods for making such bedplates.